.\"	$OpenBSD: queue.3,v 1.60 2014/09/13 01:09:31 guenther Exp $
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.\"	@(#)queue.3	8.1 (Berkeley) 12/13/93
.\"
.Dd $Mdocdate: September 13 2014 $
.Dt QUEUE 3
.Os
.Sh NAME
.Nm SLIST_ENTRY ,
.Nm SLIST_HEAD ,
.Nm SLIST_HEAD_INITIALIZER ,
.Nm SLIST_FIRST ,
.Nm SLIST_NEXT ,
.Nm SLIST_EMPTY ,
.Nm SLIST_FOREACH ,
.Nm SLIST_FOREACH_SAFE ,
.Nm SLIST_INIT ,
.Nm SLIST_INSERT_AFTER ,
.Nm SLIST_INSERT_HEAD ,
.Nm SLIST_REMOVE_AFTER ,
.Nm SLIST_REMOVE_HEAD ,
.Nm SLIST_REMOVE ,
.Nm LIST_ENTRY ,
.Nm LIST_HEAD ,
.Nm LIST_HEAD_INITIALIZER ,
.Nm LIST_FIRST ,
.Nm LIST_NEXT ,
.Nm LIST_EMPTY ,
.Nm LIST_FOREACH ,
.Nm LIST_FOREACH_SAFE ,
.Nm LIST_INIT ,
.Nm LIST_INSERT_AFTER ,
.Nm LIST_INSERT_BEFORE ,
.Nm LIST_INSERT_HEAD ,
.Nm LIST_REMOVE ,
.Nm LIST_REPLACE ,
.Nm SIMPLEQ_ENTRY ,
.Nm SIMPLEQ_HEAD ,
.Nm SIMPLEQ_HEAD_INITIALIZER ,
.Nm SIMPLEQ_FIRST ,
.Nm SIMPLEQ_NEXT ,
.Nm SIMPLEQ_EMPTY ,
.Nm SIMPLEQ_FOREACH ,
.Nm SIMPLEQ_FOREACH_SAFE ,
.Nm SIMPLEQ_INIT ,
.Nm SIMPLEQ_INSERT_AFTER ,
.Nm SIMPLEQ_INSERT_HEAD ,
.Nm SIMPLEQ_INSERT_TAIL ,
.Nm SIMPLEQ_REMOVE_AFTER ,
.Nm SIMPLEQ_REMOVE_HEAD ,
.Nm TAILQ_ENTRY ,
.Nm TAILQ_HEAD ,
.Nm TAILQ_HEAD_INITIALIZER ,
.Nm TAILQ_FIRST ,
.Nm TAILQ_NEXT ,
.Nm TAILQ_LAST ,
.Nm TAILQ_PREV ,
.Nm TAILQ_EMPTY ,
.Nm TAILQ_FOREACH ,
.Nm TAILQ_FOREACH_SAFE ,
.Nm TAILQ_FOREACH_REVERSE ,
.Nm TAILQ_FOREACH_REVERSE_SAFE ,
.Nm TAILQ_INIT ,
.Nm TAILQ_INSERT_AFTER ,
.Nm TAILQ_INSERT_BEFORE ,
.Nm TAILQ_INSERT_HEAD ,
.Nm TAILQ_INSERT_TAIL ,
.Nm TAILQ_REMOVE ,
.Nm TAILQ_REPLACE
.Nd implementations of singly-linked lists, doubly-linked lists, simple queues, and tail queues
.Sh SYNOPSIS
.In sys/queue.h
.Pp
.Fn SLIST_ENTRY "TYPE"
.Fn SLIST_HEAD "HEADNAME" "TYPE"
.Fn SLIST_HEAD_INITIALIZER "SLIST_HEAD head"
.Ft "struct TYPE *"
.Fn SLIST_FIRST "SLIST_HEAD *head"
.Ft "struct TYPE *"
.Fn SLIST_NEXT "struct TYPE *listelm" "FIELDNAME"
.Ft int
.Fn SLIST_EMPTY "SLIST_HEAD *head"
.Fn SLIST_FOREACH "VARNAME" "SLIST_HEAD *head" "FIELDNAME"
.Fn SLIST_FOREACH_SAFE "VARNAME" "SLIST_HEAD *head" "FIELDNAME" "TEMP_VARNAME"
.Ft void
.Fn SLIST_INIT "SLIST_HEAD *head"
.Ft void
.Fn SLIST_INSERT_AFTER "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SLIST_INSERT_HEAD "SLIST_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SLIST_REMOVE_AFTER "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SLIST_REMOVE_HEAD "SLIST_HEAD *head" "FIELDNAME"
.Ft void
.Fn SLIST_REMOVE "SLIST_HEAD *head" "struct TYPE *elm" "TYPE" "FIELDNAME"
.Pp
.Fn LIST_ENTRY "TYPE"
.Fn LIST_HEAD "HEADNAME" "TYPE"
.Fn LIST_HEAD_INITIALIZER "LIST_HEAD head"
.Ft "struct TYPE *"
.Fn LIST_FIRST "LIST_HEAD *head"
.Ft "struct TYPE *"
.Fn LIST_NEXT "struct TYPE *listelm" "FIELDNAME"
.Ft int
.Fn LIST_EMPTY "LIST_HEAD *head"
.Fn LIST_FOREACH "VARNAME" "LIST_HEAD *head" "FIELDNAME"
.Fn LIST_FOREACH_SAFE "VARNAME" "LIST_HEAD *head" "FIELDNAME" "TEMP_VARNAME"
.Ft void
.Fn LIST_INIT "LIST_HEAD *head"
.Ft void
.Fn LIST_INSERT_AFTER "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn LIST_INSERT_BEFORE "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn LIST_INSERT_HEAD "LIST_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn LIST_REMOVE "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn LIST_REPLACE "struct TYPE *elm" "struct TYPE *elm2" "FIELDNAME"
.Pp
.Fn SIMPLEQ_ENTRY "TYPE"
.Fn SIMPLEQ_HEAD "HEADNAME" "TYPE"
.Fn SIMPLEQ_HEAD_INITIALIZER "SIMPLEQ_HEAD head"
.Ft "struct TYPE *"
.Fn SIMPLEQ_FIRST "SIMPLEQ_HEAD *head"
.Ft "struct TYPE *"
.Fn SIMPLEQ_NEXT "struct TYPE *listelm" "FIELDNAME"
.Ft int
.Fn SIMPLEQ_EMPTY "SIMPLEQ_HEAD *head"
.Fn SIMPLEQ_FOREACH "VARNAME" "SIMPLEQ_HEAD *head" "FIELDNAME"
.Fn SIMPLEQ_FOREACH_SAFE "VARNAME" "SIMPLEQ_HEAD *head" "FIELDNAME" "TEMP_VARNAME"
.Ft void
.Fn SIMPLEQ_INIT "SIMPLEQ_HEAD *head"
.Ft void
.Fn SIMPLEQ_INSERT_AFTER "SIMPLEQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SIMPLEQ_INSERT_HEAD "SIMPLEQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SIMPLEQ_INSERT_TAIL "SIMPLEQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SIMPLEQ_REMOVE_AFTER "SIMPLEQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn SIMPLEQ_REMOVE_HEAD "SIMPLEQ_HEAD *head" "FIELDNAME"
.Pp
.Fn TAILQ_ENTRY "TYPE"
.Fn TAILQ_HEAD "HEADNAME" "TYPE"
.Fn TAILQ_HEAD_INITIALIZER "TAILQ_HEAD head"
.Ft "struct TYPE *"
.Fn TAILQ_FIRST "TAILQ_HEAD *head"
.Ft "struct TYPE *"
.Fn TAILQ_NEXT "struct TYPE *listelm" "FIELDNAME"
.Ft "struct TYPE *"
.Fn TAILQ_LAST "TAILQ_HEAD *head" "HEADNAME"
.Ft "struct TYPE *"
.Fn TAILQ_PREV "struct TYPE *listelm" "HEADNAME" "FIELDNAME"
.Ft int
.Fn TAILQ_EMPTY "TAILQ_HEAD *head"
.Fn TAILQ_FOREACH "VARNAME" "TAILQ_HEAD *head" "FIELDNAME"
.Fn TAILQ_FOREACH_SAFE "VARNAME" "TAILQ_HEAD *head" "FIELDNAME" "TEMP_VARNAME"
.Fn TAILQ_FOREACH_REVERSE "VARNAME" "TAILQ_HEAD *head" "HEADNAME" "FIELDNAME"
.Fn TAILQ_FOREACH_REVERSE_SAFE "VARNAME" "TAILQ_HEAD *head" "HEADNAME" "FIELDNAME" "TEMP_VARNAME"
.Ft void
.Fn TAILQ_INIT "TAILQ_HEAD *head"
.Ft void
.Fn TAILQ_INSERT_AFTER "TAILQ_HEAD *head" "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn TAILQ_INSERT_BEFORE "struct TYPE *listelm" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn TAILQ_INSERT_HEAD "TAILQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn TAILQ_INSERT_TAIL "TAILQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn TAILQ_REMOVE "TAILQ_HEAD *head" "struct TYPE *elm" "FIELDNAME"
.Ft void
.Fn TAILQ_REPLACE "TAILQ_HEAD *head" "struct TYPE *elm" "struct TYPE *elm2" "FIELDNAME"
.Sh DESCRIPTION
These macros define and operate on four types of data structures:
singly-linked lists, simple queues, lists, and tail queues.
All four structures support the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Insertion of a new entry at the head of the list.
.It
Insertion of a new entry after any element in the list.
.It
Removal of an entry from the head of the list.
.It
Forward traversal through the list.
.El
.Pp
Singly-linked lists are the simplest of the four data structures
and support only the above functionality.
Singly-linked lists are ideal for applications with large datasets
and few or no removals, or for implementing a LIFO queue.
.Pp
Simple queues add the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
All list insertions must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
Code size is about 15% greater and operations run about 20% slower
than singly-linked lists.
.El
.Pp
Simple queues are ideal for applications with large datasets and
few or no removals, or for implementing a FIFO queue.
.Pp
All doubly linked types of data structures (lists and tail queues)
additionally allow:
.Pp
.Bl -enum -compact -offset indent
.It
Insertion of a new entry before any element in the list.
.It
Removal of any entry in the list.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
Each element requires two pointers rather than one.
.It
Code size and execution time of operations (except for removal) is about
twice that of the singly-linked data-structures.
.El
.Pp
Lists are the simplest of the doubly linked data structures and support
only the above functionality over singly-linked lists.
.Pp
Tail queues add the following functionality:
.Pp
.Bl -enum -compact -offset indent
.It
Entries can be added at the end of a list.
.It
They may be traversed backwards, at a cost.
.El
.Pp
However:
.Pp
.Bl -enum -compact -offset indent
.It
All list insertions and removals must specify the head of the list.
.It
Each head entry requires two pointers rather than one.
.It
Code size is about 15% greater and operations run about 20% slower
than singly-linked lists.
.El
.Pp
An additional type of data structure, circular queues, violated the C
language aliasing rules and were miscompiled as a result.
All code using them should be converted to another structure;
tail queues are usually the easiest to convert to.
.Pp
In the macro definitions,
.Fa TYPE
is the name tag of a user defined structure that must contain a field of type
.Li SLIST_ENTRY ,
.Li LIST_ENTRY ,
.Li SIMPLEQ_ENTRY ,
or
.Li TAILQ_ENTRY ,
named
.Fa FIELDNAME .
The argument
.Fa HEADNAME
is the name tag of a user defined structure that must be declared
using the macros
.Fn SLIST_HEAD ,
.Fn LIST_HEAD ,
.Fn SIMPLEQ_HEAD ,
or
.Fn TAILQ_HEAD .
See the examples below for further explanation of how these macros are used.
.Sh SINGLY-LINKED LISTS
A singly-linked list is headed by a structure defined by the
.Fn SLIST_HEAD
macro.
This structure contains a single pointer to the first element on the list.
The elements are singly linked for minimum space and pointer manipulation
overhead at the expense of O(n) removal for arbitrary elements.
New elements can be added to the list after an existing element or
at the head of the list.
A
.Fa SLIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
SLIST_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fa HEADNAME
facility is often not used, leading to the following bizarre code:
.Bd -literal -offset indent
SLIST_HEAD(, TYPE) head, *headp;
.Ed
.Pp
The
.Fn SLIST_ENTRY
macro declares a structure that connects the elements in the list.
.Pp
The
.Fn SLIST_INIT
macro initializes the list referenced by
.Fa head .
.Pp
The list can also be initialized statically by using the
.Fn SLIST_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn SLIST_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the list.
.Pp
The
.Fn SLIST_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn SLIST_REMOVE_HEAD
macro removes the first element of the list pointed by
.Fa head .
.Pp
The
.Fn SLIST_REMOVE_AFTER
macro removes the list element immediately following
.Fa elm .
.Pp
The
.Fn SLIST_REMOVE
macro removes the element
.Fa elm
of the list pointed by
.Fa head .
.Pp
The
.Fn SLIST_FIRST
and
.Fn SLIST_NEXT
macros can be used to traverse the list:
.Bd -literal -offset indent
for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, FIELDNAME))
.Ed
.Pp
Or, for simplicity, one can use the
.Fn SLIST_FOREACH
macro:
.Bd -literal -offset indent
SLIST_FOREACH(np, head, FIELDNAME)
.Ed
.Pp
The macro
.Fn SLIST_FOREACH_SAFE
traverses the list referenced by head in a
forward direction, assigning each element in turn to var.
However, unlike
.Fn SLIST_FOREACH
it is permitted to remove var as well
as free it from within the loop safely without interfering with the traversal.
.Pp
The
.Fn SLIST_EMPTY
macro should be used to check whether a simple list is empty.
.Sh SINGLY-LINKED LIST EXAMPLE
.Bd -literal
SLIST_HEAD(listhead, entry) head;
struct entry {
	...
	SLIST_ENTRY(entry) entries;	/* Simple list. */
	...
} *n1, *n2, *np;

SLIST_INIT(&head);			/* Initialize simple list. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
SLIST_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
SLIST_INSERT_AFTER(n1, n2, entries);

SLIST_FOREACH(np, &head, entries)	/* Forward traversal. */
	np-> ...

while (!SLIST_EMPTY(&head)) {	 	/* Delete. */
	n1 = SLIST_FIRST(&head);
	SLIST_REMOVE_HEAD(&head, entries);
	free(n1);
}

.Ed
.Sh LISTS
A list is headed by a structure defined by the
.Fn LIST_HEAD
macro.
This structure contains a single pointer to the first element on the list.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the list.
New elements can be added to the list after an existing element,
before an existing element, or at the head of the list.
A
.Fa LIST_HEAD
structure is declared as follows:
.Bd -literal -offset indent
LIST_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the list.
A pointer to the head of the list can later be declared as:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fa HEADNAME
facility is often not used, leading to the following bizarre code:
.Bd -literal -offset indent
LIST_HEAD(, TYPE) head, *headp;
.Ed
.Pp
The
.Fn LIST_ENTRY
macro declares a structure that connects the elements in the list.
.Pp
The
.Fn LIST_INIT
macro initializes the list referenced by
.Fa head .
.Pp
The list can also be initialized statically by using the
.Fn LIST_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn LIST_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the list.
.Pp
The
.Fn LIST_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn LIST_INSERT_BEFORE
macro inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The
.Fn LIST_REMOVE
macro removes the element
.Fa elm
from the list.
.Pp
The
.Fn LIST_REPLACE
macro replaces the list element
.Fa elm
with the new element
.Fa elm2 .
.Pp
The
.Fn LIST_FIRST
and
.Fn LIST_NEXT
macros can be used to traverse the list:
.Bd -literal -offset indent
for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, FIELDNAME))
.Ed
.Pp
Or, for simplicity, one can use the
.Fn LIST_FOREACH
macro:
.Bd -literal -offset indent
LIST_FOREACH(np, head, FIELDNAME)
.Ed
.Pp
The macro
.Fn LIST_FOREACH_SAFE
traverses the list referenced by head in a
forward direction, assigning each element in turn to var.
However, unlike
.Fn LIST_FOREACH
it is permitted to remove var as well
as free it from within the loop safely without interfering with the traversal.
.Pp
The
.Fn LIST_EMPTY
macro should be used to check whether a list is empty.
.Sh LIST EXAMPLE
.Bd -literal
LIST_HEAD(listhead, entry) head;
struct entry {
	...
	LIST_ENTRY(entry) entries;	/* List. */
	...
} *n1, *n2, *np;

LIST_INIT(&head);			/* Initialize list. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
LIST_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
LIST_INSERT_AFTER(n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
LIST_INSERT_BEFORE(n1, n2, entries);
					/* Forward traversal. */
LIST_FOREACH(np, &head, entries)
	np-> ...

while (!LIST_EMPTY(&head))		/* Delete. */
	n1 = LIST_FIRST(&head);
	LIST_REMOVE(n1, entries);
	free(n1);
}
.Ed
.Sh SIMPLE QUEUES
A simple queue is headed by a structure defined by the
.Fn SIMPLEQ_HEAD
macro.
This structure contains a pair of pointers, one to the first element in the
simple queue and the other to the last element in the simple queue.
The elements are singly linked.
New elements can be added to the queue after an existing element,
at the head of the queue or at the tail of the queue.
A
.Fa SIMPLEQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
SIMPLEQ_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the queue.
A pointer to the head of the queue can later be declared as:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn SIMPLEQ_ENTRY
macro declares a structure that connects the elements in
the queue.
.Pp
The
.Fn SIMPLEQ_INIT
macro initializes the queue referenced by
.Fa head .
.Pp
The queue can also be initialized statically by using the
.Fn SIMPLEQ_HEAD_INITIALIZER
macro like this:
.Bd -literal -offset indent
SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);
.Ed
.Pp
The
.Fn SIMPLEQ_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn SIMPLEQ_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the queue.
.Pp
The
.Fn SIMPLEQ_INSERT_TAIL
macro inserts the new element
.Fa elm
at the end of the queue.
.Pp
The
.Fn SIMPLEQ_REMOVE_AFTER
macro removes the queue element immediately following
.Fa elm .
.Pp
The
.Fn SIMPLEQ_REMOVE_HEAD
macro removes the first element
from the queue.
.Pp
The
.Fn SIMPLEQ_FIRST
and
.Fn SIMPLEQ_NEXT
macros can be used to traverse the queue.
The
.Fn SIMPLEQ_FOREACH
is used for queue traversal:
.Bd -literal -offset indent
SIMPLEQ_FOREACH(np, head, FIELDNAME)
.Ed
.Pp
The macro
.Fn SIMPLEQ_FOREACH_SAFE
traverses the queue referenced by head in a
forward direction, assigning each element in turn to var.
However, unlike
.Fn SIMPLEQ_FOREACH
it is permitted to remove var as well
as free it from within the loop safely without interfering with the traversal.
.Pp
The
.Fn SIMPLEQ_EMPTY
macro should be used to check whether a list is empty.
.Sh SIMPLE QUEUE EXAMPLE
.Bd -literal
SIMPLEQ_HEAD(listhead, entry) head = SIMPLEQ_HEAD_INITIALIZER(head);
struct entry {
	...
	SIMPLEQ_ENTRY(entry) entries;	/* Simple queue. */
	...
} *n1, *n2, *np;

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
SIMPLEQ_INSERT_HEAD(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
SIMPLEQ_INSERT_AFTER(&head, n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert at the tail. */
SIMPLEQ_INSERT_TAIL(&head, n2, entries);
					/* Forward traversal. */
SIMPLEQ_FOREACH(np, &head, entries)
	np-> ...
					/* Delete. */
while (!SIMPLEQ_EMPTY(&head)) {
	n1 = SIMPLEQ_FIRST(&head);
	SIMPLEQ_REMOVE_HEAD(&head, entries);
	free(n1);
}
.Ed
.Sh TAIL QUEUES
A tail queue is headed by a structure defined by the
.Fn TAILQ_HEAD
macro.
This structure contains a pair of pointers,
one to the first element in the tail queue and the other to
the last element in the tail queue.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the tail queue.
New elements can be added to the queue after an existing element,
before an existing element, at the head of the queue, or at the end
of the queue.
A
.Fa TAILQ_HEAD
structure is declared as follows:
.Bd -literal -offset indent
TAILQ_HEAD(HEADNAME, TYPE) head;
.Ed
.Pp
where
.Fa HEADNAME
is the name of the structure to be defined, and struct
.Fa TYPE
is the type of the elements to be linked into the tail queue.
A pointer to the head of the tail queue can later be declared as:
.Bd -literal -offset indent
struct HEADNAME *headp;
.Ed
.Pp
(The names
.Li head
and
.Li headp
are user selectable.)
.Pp
The
.Fn TAILQ_ENTRY
macro declares a structure that connects the elements in
the tail queue.
.Pp
The
.Fn TAILQ_INIT
macro initializes the tail queue referenced by
.Fa head .
.Pp
The tail queue can also be initialized statically by using the
.Fn TAILQ_HEAD_INITIALIZER
macro.
.Pp
The
.Fn TAILQ_INSERT_HEAD
macro inserts the new element
.Fa elm
at the head of the tail queue.
.Pp
The
.Fn TAILQ_INSERT_TAIL
macro inserts the new element
.Fa elm
at the end of the tail queue.
.Pp
The
.Fn TAILQ_INSERT_AFTER
macro inserts the new element
.Fa elm
after the element
.Fa listelm .
.Pp
The
.Fn TAILQ_INSERT_BEFORE
macro inserts the new element
.Fa elm
before the element
.Fa listelm .
.Pp
The
.Fn TAILQ_REMOVE
macro removes the element
.Fa elm
from the tail queue.
.Pp
The
.Fn TAILQ_REPLACE
macro replaces the list element
.Fa elm
with the new element
.Fa elm2 .
.Pp
.Fn TAILQ_FOREACH
and
.Fn TAILQ_FOREACH_REVERSE
are used for traversing a tail queue.
.Fn TAILQ_FOREACH
starts at the first element and proceeds towards the last.
.Fn TAILQ_FOREACH_REVERSE
starts at the last element and proceeds towards the first.
.Bd -literal -offset indent
TAILQ_FOREACH(np, &head, FIELDNAME)
TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, FIELDNAME)
.Ed
.Pp
The macros
.Fn TAILQ_FOREACH_SAFE
and
.Fn TAILQ_FOREACH_REVERSE_SAFE
traverse the list referenced by head
in a forward or reverse direction respectively,
assigning each element in turn to var.
However, unlike their unsafe counterparts,
they permit both the removal of var
as well as freeing it from within the loop safely
without interfering with the traversal.
.Pp
The
.Fn TAILQ_FIRST ,
.Fn TAILQ_NEXT ,
.Fn TAILQ_LAST
and
.Fn TAILQ_PREV
macros can be used to manually traverse a tail queue or an arbitrary part of
one.
.Pp
The
.Fn TAILQ_EMPTY
macro should be used to check whether a tail queue is empty.
.Sh TAIL QUEUE EXAMPLE
.Bd -literal
TAILQ_HEAD(tailhead, entry) head;
struct entry {
	...
	TAILQ_ENTRY(entry) entries;	/* Tail queue. */
	...
} *n1, *n2, *np;

TAILQ_INIT(&head);			/* Initialize queue. */

n1 = malloc(sizeof(struct entry));	/* Insert at the head. */
TAILQ_INSERT_HEAD(&head, n1, entries);

n1 = malloc(sizeof(struct entry));	/* Insert at the tail. */
TAILQ_INSERT_TAIL(&head, n1, entries);

n2 = malloc(sizeof(struct entry));	/* Insert after. */
TAILQ_INSERT_AFTER(&head, n1, n2, entries);

n2 = malloc(sizeof(struct entry));	/* Insert before. */
TAILQ_INSERT_BEFORE(n1, n2, entries);
					/* Forward traversal. */
TAILQ_FOREACH(np, &head, entries)
	np-> ...
					/* Manual forward traversal. */
for (np = n2; np != NULL; np = TAILQ_NEXT(np, entries))
	np-> ...
					/* Delete. */
while ((np = TAILQ_FIRST(&head))) {
	TAILQ_REMOVE(&head, np, entries);
	free(np);
}

.Ed
.Sh NOTES
It is an error to assume the next and previous fields are preserved
after an element has been removed from a list or queue.
Using any macro (except the various forms of insertion) on an element
removed from a list or queue is incorrect.
An example of erroneous usage is removing the same element twice.
.Pp
The
.Fn SLIST_END ,
.Fn LIST_END ,
.Fn SIMPLEQ_END
and
.Fn TAILQ_END
macros are deprecated; they provided symmetry with the historical
.Fn CIRCLEQ_END
and just expand to
.Dv NULL .
.Pp
Trying to free a list in the following way is a common error:
.Bd -literal -offset indent
LIST_FOREACH(var, head, entry)
	free(var);
free(head);
.Ed
.Pp
Since
.Va var
is free'd, the FOREACH macros refer to a pointer that may have been
reallocated already.
A similar situation occurs when the current element is deleted
from the list.
In cases like these the data structure's FOREACH_SAFE macros should be used
instead.
.Sh HISTORY
The
.Nm queue
functions first appeared in
.Bx 4.4 .
The historical circle queue macros were deprecated in
.Ox 5.5 .
